The present invention relates to a semiconductor device, and more particularly, to a semiconductor device that converts an AC output of an AC generator to a DC output.
With regard to a rectification device for an AC generator, in order to obtain a semiconductor device whose electrical characteristics will not deteriorate for a long period of time, even in a severe environment in which a large number of thermal shocks are repeatedly applied, JP-A-4-229639 proposes a structure in which a semiconductor chip is encapsulated with an epoxy-based insulating material. This encapsulated structure is intended to secure a necessary current-carrying path by utilizing contraction of the resin after molding to apply a pressure, in a direction perpendicular to a bonded plane of a semiconductor chip, to a case electrode even if cracking occurs in a bonding material for the case electrode and semiconductor chip. Furthermore, as a similar invention, JP-A-10-215552 proposes a structure in which a groove is provided around a semiconductor chip. The groove is filled with an insulating resin material at a high pressure exceeding an atmospheric pressure, and is then molded in such a way that a residual compressive stress is produced in the insulating resin material.
When a semiconductor device is mounted in a location such as an engine compartment of an automobile, there is quite a large influence such as high temperatures and an increase in an amount of heat produced by a generator due to variations of electric load on a vehicle side. Especially, automobiles present a severe environment subject to repetitive cooling and heating extending over a wide temperature range caused by temperature differences between summer and winter, etc. Therefore automobiles require semiconductor devices having an excellent heat radiation characteristics and thermal fatigue resistances.
When a semiconductor device repeatedly receives a large number of heat shocks, distortion caused by a difference in coefficients of linear thermal expansion among the components making up the semiconductor device is applied to its bonding material (for example, solder). If sufficient counter measures are not provided, cracks may occur in this bonding material. Once cracks occur, a cross-sectional area of the bonding material that constitutes a current-carrying path is reduced and electrical resistance increases. As a result, heat generation is increased and an amount of heat radiation through the bonding material is reduced. Accordingly, temperature of the semiconductor chip tends to abnormally increase. Thus, if the temperature reaches a point of melting of the bonding material, or if the semiconductor chip reaches a limit of heat resistance, the rectification function may be lost.
The inventors of the present invention have found that the structures described in the above-mentioned publications are not sufficient in order to obtain semiconductor devices which can fully cope with such an operating environment and effectively achieves improvement of thermal fatigue life and heat radiation characteristics.
According to the structure described in JP-A-4-229639, it is possible to suppress abnormal temperature rises of a semiconductor chip to a certain degree by suppressing an increase of electrical resistance, and to support the amount of heat generation by securing a current-carrying path. However, this resin-encapsulated structure is only intended to apply a pressure to the case electrode by utilizing contraction of the resin after molding, and, therefore is not originally intended to reduce distortion of the bonding material caused by differences in coefficients of linear thermal expansion. Therefore, the structure has a certain degree of effectiveness in extending service life, but cannot totally solve the problem of generation of distortion of the bonding material.
In the structure described in JP-A-10-215552, there is no reduction of elastic modulus of the insulating resin material even at a high temperature (140° C.). Therefore, it is possible to prevent reduction of the effect of suppressing free expansion of the case electrode due to reduction of the elastic modulus. However, there is a problem of the life of the insulating resin material itself, such as deterioration of the insulating material, since the rigidity of the insulating resin material is high. Furthermore, since the semiconductor chip is located in a place higher than the radiating fins of the structure, it results in high heat resistance and insufficient heat radiation characteristics.
JP-A-11-243165 discloses a structure provided with a ring-shaped groove having a dovetail cross section near a center between a semiconductor chip and a wall inside a concave support electrode (case electrode). The groove is assumed to be placed near the center between the semiconductor chip and the wall in order to assist in preventing the resin filled in the concave support electrode from separating from the support electrode.
Thus, it is an object of the present invention to provide a semiconductor device capable of preventing cracking in a semiconductor chip caused by a difference in mutual thermal deformation between a case electrode and a semiconductor chip electrically bonded together by using a bonding material, as well as being capable of reducing distortion of the bonding material and improving a heat radiation characteristic of the semiconductor chip.